the flushing cap you refer to is fine for new installs.
the flushing cross is more for repairs.
we use ours all the time when we replace broken nozzles, clean clogged screens, etc...
they also make awesome riser extractors when the risor slpis out of your hand and shoots down inside the body ( especially toro and other male thread risers.

We carry several pairs and can clip them on as many as 6 sprinklers at a time. Remove nozzles/filters, blow them out, insert new filter/nozzle, turn on zone with remote and adjust. Move along to next problem. Work great especially when we're doing a complete walkthrough on a system.